As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand for batteries has also sharply increased as an energy source for the mobile devices. Also, much research on secondary batteries satisfying various needs has been carried out.
A secondary battery is constructed in a structure in which a chargeable and dischargeable electrode assembly of a cathode/separator/anode structure is mounted in a battery case. A representative example of the electrode assembly is a jelly-roll type electrode assembly.
A jelly-roll type assembly (or a ‘jelly-roll’) is manufactured by applying, drying, and pressing electrode active materials to opposite major surfaces of metal sheets, which are used as current collectors, cutting into the shape of a band having a predetermined width and length, and winding the metal sheets, serving as an anode and a cathode, while disposing a separator between the anode and the cathode, in a helical shape.
During the manufacture of the jelly-roll, however, the radius of curvature of the inner sheet is small at the central region of the jelly-roll, with the result that the active material applied toward the inner surface side of the jelly roll is pressurized, during the winding of the sheets, and therefore, the active material lumps or the density of the active material greatly increases. Due to the nonuniform distribution of the active material, the active material layers inside and outside each current collector have different electrochemical reaction amounts, with the result that jelly-roll twist occurs, and therefore, the service life of the battery decreases.
In a lithium secondary battery, which is a representative example of secondary batteries, some electrolyte containing lithium salt is decomposed through side reaction, during the repetitive charge and discharge of the secondary battery, with the result that the amount of the electrolyte gradually decreases.
When the battery is overcharged with a voltage of approximately 4.5 V or more, for example, a cathode active material is decomposed, lithium metal grows at an anode in the shape of a dendrite, and an electrolyte is decomposed. At this time, heat is generated, and therefore, the above-described decomposition process and several side reactions rapidly progress, with the result that the electrolyte is consumed. This phenomenon is more accelerated at the inside active material layer of the jelly-roll where the electrochemical reaction is more active. Also, the nonuniform consumption of the electrolyte at the outside region and the inside region of the jelly-roll causes the jelly-roll twist.
In connection with this matter, Korean Patent Application Publication No. 2004-089519 discloses a technology for maintaining the capacity ratio of an active material layer formed inside an anode sheet of a jelly-roll to an active material layer formed outside the anode sheet of the jelly-roll 0.6 to 0.8 to prevent current collectors from being cut and wrinkle. According to the disclosed technology, the amount of active material on the inner surface of the wound jelly-roll is less at the central region of the jelly-roll, where the radius of curvature is small, than at the outermost region of the jelly-roll, and therefore, winding of the sheet is easy, and the wrinkles preventing effect is excellent.
However, the inventors of the present invention have found that the easy winding and wrinkles preventing effect, which are effects acquired by the capacity ratio between the active material layers, are lowered due to the increase in radius of curvature as the outermost region of the jelly-roll is reached; rather, the capacity of the battery is decreased, and the service life of the battery is reduced, due to unbalance of the active material layers at the outer and inner surfaces of the wound jelly-roll.
Consequently, there is a high necessity for a technology that is capable of fundamentally solving the above-mentioned problems.